How do I create a OS with assembly? I'm new to fasm, don't know how to make OSes with it yet.

If you're a beginner, start with something simpler.

You need first to understand the hardware micro-architecture of CPUs.

That said, I suggest you look at RVA23, aka RISC-V since this is a 'cleaned up' micro-architecture (still with a lot of not really pertinent features, that to increase likelyness with the current mainstream full of technical debt micro-architectures).

I'm also a beginner, but I would recommend first learning how to create a solid bootloader, then get simple hardware information before anything else

Then a choice has to be made: embedded uses static device trees, but PCs use UEFI/ACPI.

Not necessarily. I've created a single library that handles both: hwdet. No matter the input format, this provides a common output.

It's a single C header file, no memory allocation, no dependencies (not even libc), so should be trivial to port to Assembly (sorry about that, I've used C because I wanted it to be portable).
It handles about 99% of all DSDT I could find (including real machines) and actual RaspberryPi DTBs as well. It does not execute AML on-the-fly for security reasons, only interprets it to parse the hardware resources.

You can also find the full AML and FDT specifications in the repo if you want to implement these in Assembly from ground up.

You must run some ACPI byte code if you want the hardware properly initialized, or even some hardware configuration element can only be done with running that byte code.

I remember Linus T. about it, "designed by monkeys high on LSD".

Well, this summerize the state of hardware/software nowadays.

Not necessarily. I put lot of effort into my lib to interpret the AML structures natively as much as possible instead letting the bytecode do that to mitigate the security risks. You can get pretty far without bytecode. Yeah, this s*xx big time, not easy to implement, but doable. TBH, in reality there's no logic in AML that's not already implemented by device drivers.

Even if everything were shiny, you usually still can't run the provided bytecode because lots of machine's AML is buggy like hell. (On a side note, I guess the armada of buggy factory default AMLs is the main reason why the device drivers implement the required logic anyway, imho).

He was absolutely right. A hardware configuration should not need any virtual machine in the first place. I mean FDT works without it just fine.

Sadly I must agree.

I think ACPI tables are the last thing an OS developer needs, once all the basic functionality is ready. That is, these are small, albeit important, details, such as power management for PNP devices.

During the POST process, any hardware must report its system requirements, and the BIOS stores them in the PCI-Config-Space table. These include the base addresses of devices in memory, their I/O port numbers, and so on, which are required by the system drivers. Many motherboard controllers also have hard-coded memory addresses, such as IO_APIC=0xFEC00000, LAPIC=0xFEE00000, as well as the first 256 CPU ports. All of this facilitates the collection of information about physical devices.